Double-fed asynchronous machines in the power range from 20 MVA to over 500 MVA can be used for variable-speed energy production. These machines are distinguished by a distributed three-phase winding on the rotor. The rotor winding consists of individual bars which are embedded in slots in the rotor laminations. The individual bars are connected in the winding head to form a winding. The currents are fed in by at least three collector rings which are fixed to the shaft at the end of the machine. A section of such an asynchronous machine is reproduced in highly simplified form in FIG. 1. The asynchronous machine 10 shown in FIG. 1 has a machine axis 13. Rotatably about this axis 13 is a central body 11 with a shaft, on which the collector rings 12 are arranged. The rotor lamination stack 14, to which an auxiliary rim 20 is connected below a winding head 16 of the rotor winding, is arranged around the central body 11. The rotor lamination stack 14 is encompassed concentrically by a stator lamination stack 15 in which is mounted a stator winding, a stator winding head 17 of which projects outwards at the end of the body. The rotor lamination stack 14 is shown enlarged in section in FIG. 2.
As the rotors of double-fed asynchronous machines carry a rotor winding 18, said winding must be safeguarded against the centrifugal forces which occur. On the one hand, the rotor lamination stack serves to absorb these forces and at the same time defines the path for the magnetic flux. The auxiliary rim 20 serves to absorb the centrifugal forces which act on the rotor winding head 16. The auxiliary rim 20, like the rotor lamination stack 14, consists of laminated sheets which are pressed in the axial direction to form a composite assembly. In doing so, a pressing plate 19 which distributes the compression force applied by the bolts 21, 22 to the sheets of the rotor lamination stack can be used (see, for example, DE-A1-195 13 457 or DE-A1-10 2007 000 668).
Different requirements are imposed on the rotor lamination stack 14. The principle of a subdivision into an electrical part 14a and a mechanical part 14b is shown in FIG. 2. On the one hand, there must be sufficient axial force between the layers of sheets in the teeth to guarantee the uniformity of the body. In order to avoid vibrations, the layers must not come loose, as relative movements between the teeth and the rotor winding 18 could damage the insulation. On the other hand, the pressure must not be too high in order to avoid damage to the insulation layers between the individual sheets, as such damage would lead to increased losses. The axial force must be higher in the mechanical part 14b of the rim than in the electrical part 14a in order to obtain a certain frictional force between the sheets.